Optical structures including nanocrystals

ABSTRACT

An optical structure can include a nanocrystal on a surface of an optical waveguide in a manner to couple the nanocrystal to the optical field of light propagating through the optical waveguide to generate an emission from the nanocrystal.

CLAIM OF PRIORITY

This application claims priority under 35 USC 371 to InternationalApplication No. PCT/US2007/012040, filed on May 21, 2007, which claimspriority to U.S. Provisional Application Ser. No. 60/747,805, filed onMay 21, 2006, each of which is incorporated by reference in itsentirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant NumberFA9550-04-1-0462 awarded by the Air Force. The government has certainrights to the invention.

TECHNICAL FIELD

The invention relates to optical structures including nanocrystals.

BACKGROUND

Optical waveguides, such as fibers and planar waveguides, which takeadvantage of total internal reflection have been used in a wide range ofsensing, communication, and illumination applications. Light can bedelivered through optical fibers with great efficiency over longdistances because of the perfect mirroring that is provided by thecore/cladding dielectric index step interface. Typically, the opticalfield in an optical fiber element is entirely confined because of thiscore/cladding interface.

SUMMARY

An optical structure can include a nanocrystal on a surface of anoptical waveguide in a manner to couple the nanocrystal to the opticalfield of light propagating through the optical waveguide to generate anemission from the nanocrystal. For example, one or more semiconductornanocrystals, or quantum dots, can be placed in the vicinity of anoptical structure such as a waveguide, for example, a fiber opticelement. The optical field of light which is propagating through thewaveguide can couple with the nanocrystal and cause the nanocrystal toemit light.

Advantageously, the light emitting structure can allow for thestraightforward and efficient distribution of an excitation light sourceand coupling to a highly efficient downconverting element which will beuseful for a range of lighting applications, including optical displays,sensors, and other applications. The light emitting structure can haveparticular relevance to solid state lighting applications. An excitationsource can be used to efficiently distribute excitation wavelengththrough the waveguide and downconverted at the point of use to anappropriate spectral composition by applying the right combination ofdownconverting elements, including nanocrystals. Nanocrystals are anespecially appropriate material set because of their broad spectraltunability, long lifetime in photoluminescence (far exceeding that oforganic dyes), and easy solution processability.

The nanocrystal can be a semiconductor nanocrystal. The semiconductornanocrystal includes a core including a first semiconductor material.The semiconductor nanocrystal can include an overcoating on a surface ofthe core including a second semiconductor material. The semiconductornanocrystal can include an outer layer including a compound linked to asurface of the nanocrystal.

In one aspect, an optical structure includes a nanocrystal on a surfaceof an optical waveguide, the nanocrystal being positioned to beoptically coupled to an optical field propagating through the opticalwaveguide.

In another aspect, a light emitting structure includes a light sourcearranged to introduce light including an excitation wavelength into anoptical waveguide, and a nanocrystal on a surface of the opticalwaveguide, the nanocrystal being positioned to be optically coupled toan optical field propagating through the optical waveguide and capableof absorbing the excitation wavelength of light and emitting an emissionwavelength of light.

In another aspect, a method of producing light includes introducinglight from a light source including an excitation wavelength into anoptical waveguide, the excitation wavelength propagating through theoptical waveguide and optically coupling to a nanocrystal on a surfaceof the optical waveguide, the nanocrystal absorbing the excitationwavelength and emitting an emission wavelength from the surface.

In another aspect, a method of making an optical structure includesplacing a nanocrystal on a surface of an optical waveguide in a positionto optically couple the nanocrystal to an optical field propagatingthrough the optical waveguide.

The waveguide can be an optical fiber or a planar waveguide. The opticalfiber can have a cladding layer that allows light to escape at aselected amount along the length of the fiber. The nanocrystal can be asemiconductor nanocrystal. The semiconductor nanocrystal can include acore including a first semiconductor material. The semiconductornanocrystal can include an overcoating on a surface of the coreincluding a second semiconductor material.

A plurality of nanocrystals can be distributed at a first portion of thesurface. A plurality of nanocrystals can be distributed at a secondportion of the surface. The plurality of nanocrystals distributed at thefirst portion of the surface can have a composition different from theplurality of nanocrystals distributed at the first portion of thesurface. The plurality of nanocrystals distributed at the first portionof the surface has an emission wavelength different from the pluralityof nanocrystals distributed at the first portion of the surface.

The surface of the optical waveguide can be modified to increasecoupling between the optical field and the nanocrystal to allow light toescape at a selected amount at selected locations. The excitationwavelength can propagate through the optical waveguide and opticallycouple to a plurality of nanocrystals on a first portion of a surface ofthe optical waveguide. The excitation wavelength propagates through theoptical waveguide and optically couple to a plurality of nanocrystals ona second portion of the surface.

The nanocrystal can be placed on the surface by dip coating, spincoating, painting or printing. The surface of the optical waveguide canbe processed prior to placing the nanocrystal.

Other features, objects and advantages will be apparent from thedescription, the drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an optical structure includingnanocrystals.

FIG. 2 is a schematic illustration of an optical structure includingnanocrystals viewed from the top and from the side.

FIG. 3 is a graph displaying a light emission from an optical structureincluding nanocrystals.

FIG. 4 is a photograph illustrating light emission from an opticalstructure including nanocrystals.

DETAILED DESCRIPTION

A light emitting structure can include a nanocrystal on a surface of anoptical structure. The nanocrystal is coupled to the optical field oflight propagating through the optical structure. For example, one ormore semiconductor nanocrystals, or quantum dots, can be placed in thevicinity of an optical structure such as a waveguide, for example, afiber optic element. In one example, a portion of the surface of thewaveguide is coated with a thin layer of nanocrystals. The thin layercan be a monolayer or a multilayer. The optical field of light which ispropagating through the waveguide can couple with the nanocrystal andcause them to emit light at an emission wavelength.

The layer has a thickness sufficient to generate a desired amount oflight at the emission wavelength, and is thin enough to avoidsignificant self absorption of the emission wavelength. The compositionand thickness of the nanocrystal layer, and the size, and distributionof sizes of the individual nanocrystals in the layer can be selected togenerate a particular emission wavelength profile from each particularportion of the surface of the waveguide. In addition, the confinement ofthe propagating excitation wavelength of light provided by the waveguidecan be tuned, for example by modifying the structure of the surface ofthe waveguide or thickness of the waveguide, to select the amount ofexcitation wavelength the nanocrystal will encounter at differentpositions along the surface. For example, it is possible to thin orremove portions of the cladding layer of a core-cladding optical fiberto couple the light propagating inside the fiber to materials which havebeen placed on its surface. This occurs because the optical fieldpenetrates beyond the core/cladding or core/air interface a very smalldistance. The resulting evanescent optical field can be used to excitethe nanocrystal on the surface of the waveguide with the light which isnormally otherwise confined to the fiber.

The light emitted from the nanocrystals on different portions of thewaveguide surface can generate a variety of colors and intensity levels,making the light emitting structure useful in a broad range of lightingapplications, such as, for example, solid state lighting applications.An efficient excitation wavelength source can be distributed through thewaveguide and downconverted at the point of use to an appropriatespectral composition by applying the right combination of downconvertingelements at the surface of the waveguide, for example, a nanocrystal orcombination of nanocrystals. Nanocrystals are an especially appropriatematerial to use for the downconversion because of their broad spectraltunability, long lifetime in photoluminescence (far exceeding that oforganic dyes), and easy solution processability.

The nanocrystals can be placed on a surface of a waveguide by dipcoating, drop coating, spin coating, painting or printing thenanocrystal on the surface. Printing can include ink jet printing ormicrocontact printing. Microcontact printing and related techniques aredescribed in, for example, U.S. Pat. Nos. 5,512,131; 6,180,239; and6,518,168, each of which is incorporated by reference in its entirety.In some circumstances, the stamp can be a featureless stamp having apattern of ink, where the pattern is formed when the ink is applied tothe stamp. See U.S. patent application Ser. No. 11/253,612, filed Oct.21, 2005, which is incorporated by reference in its entirety.

Referring to FIG. 1, light emitting structure 10 includes light source20 arranged to couple light into optical waveguide 30. Light source 20can be, for example, a laser or light emitting diode that emits light ata wavelength suitable to excite the nanocrystal and cause emission, forexample, a blue light emitting diode. Nanocrystals on portions of thesurface of optical waveguide 30 form nanocrystal regions, such asregions 40, 50 and 60. In each of these regions, one or morenanocrystals, for example, nanocrystals 41 a and 41 b form a layer. Thelayer can be a monolayer or a multilayer. The nanocrystals 41 a and 41 bcan have a similar composition or size, i.e., can have a similaremission wavelength, or can have a different composition or size, i.e.,can have a different emission wavelength. In each of the regions, thenanocrystals are selected to provide particular emission wavelengths oflight, which in turn can provide different colors and intensities (orthe same) at the various positions. The nanocrystal can be, for example,a semiconductor nanocrystal. The regions 40, 50 and 60 can contain otheradditives, including dyes, pigments, organic or inorganic matrixmaterials, or other components that can help protect the regions fromdegradation. Optionally, the regions can be coated by a protectivematerial.

The waveguide can have a variety of different shapes or configurations.For example, another optical structure that can also contribute toefficient light downconversion is shown in FIG. 2. In this structure,the light is injected, for example by a blue light emitting diode (LED)in an optical waveguide coated with nanocrystals. The evanescent tail ofthe waveguide optical mode can be absorbed by the nanocrystal layer. Theblue light that is not absorbed continues circling the waveguide untileventually absorbed by nanocrystals, which in turn convert the bluelight into an emission wavelength of a different color. Again, theemission wavelength arises from the size and/or composition of thenanocrystal.

In general, the light source, such as the blue LED, can be any other LEDor other light source. In addition, any nanocrystals can be coated onthe surface of the optical structures, but only the nanocrystals thatcan absorb the excitation spectrum produced by the light source will beexcited by the light. The nanocrystal film can consist of a mixture ofdifferent nanocrystals. For example, combinations of nanocrystals can beused to generate a white light spectrum. The thickness of thenanocrystal film can be adjusted in order to optimize the spectralemission. Also, it is usually desirable to minimize nanocrystal lightself absorption which predicates use of the very thin nanocrystal films.

The semiconductor nanocrystals can have a broad absorption band with anintense, narrow band emission. The peak wavelength of emission can betuned from throughout the visible and infrared regions, depending on thesize, shape, composition, and structural configuration of thenanocrystals. The nanocrystals can be prepared with an outer surfacehaving desired chemical characteristics (such as a desired solubility).Light emission by nanocrystals can be stable for long periods of time.

When a nanocrystal achieves an excited state (or in other words, anexciton is located on the nanocrystal), emission can occur at anemission wavelength. The emission has a frequency that corresponds tothe band gap of the quantum confined semiconductor material. The bandgap is a function of the size of the nanocrystal. Nanocrystals havingsmall diameters can have properties intermediate between molecular andbulk forms of matter. For example, nanocrystals based on semiconductormaterials having small diameters can exhibit quantum confinement of boththe electron and hole in all three dimensions, which leads to anincrease in the effective band gap of the material with decreasingcrystallite size. Consequently, both the optical absorption and emissionof nanocrystals shift to the blue, or to higher energies, as the size ofthe crystallites decreases.

The emission from the nanocrystal can be a narrow Gaussian emission bandthat can be tuned through the complete wavelength range of theultraviolet, visible, or infrared regions of the spectrum by varying thesize of the nanocrystal, the composition of the nanocrystal, or both.For example, CdSe can be tuned in the visible region and InAs can betuned in the infrared region. The narrow size distribution of apopulation of nanocrystals can result in emission of light in a narrowspectral range. The population can be monodisperse and can exhibit lessthan a 15% rms deviation in diameter of the nanocrystals, preferablyless than 10%, more preferably less than 5%. Spectral emissions in anarrow range of no greater than about 75 nm, preferably 60 nm, morepreferably 40 nm, and most preferably 30 nm full width at half max(FWHM) for nanocrystals that emit in the visible can be observed.IR-emitting nanocrystals can have a FWHM of no greater than 150 nm, orno greater than 100 nm. Expressed in terms of the energy of theemission, the emission can have a FWHM of no greater than 0.05 eV, or nogreater than 0.03 eV. The breadth of the emission decreases as thedispersity of nanocrystal diameters decreases. Semiconductornanocrystals can have high emission quantum efficiencies such as greaterthan 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80%.

The semiconductor forming the nanocrystals can include a Group II-VIcompound, a Group II-V compound, a Group III-VI compound, a Group III-Vcompound, a Group IV-VI compound, a Group I-III-VI compound, a GroupII-IV-VI compound, or a Group II-IV-V compound, for example, ZnO, ZnS,ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe,HgTe, AIN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb,TlN, TlP, TlAs, TlSb, TlSb, PbS, PbSe, PbTe, or mixtures thereof.

Methods of preparing monodisperse semiconductor nanocrystals includepyrolysis of organometallic reagents, such as dimethyl cadmium, injectedinto a hot, coordinating solvent. This permits discrete nucleation andresults in the controlled growth of macroscopic quantities ofnanocrystals. Preparation and manipulation of nanocrystals aredescribed, for example, in U.S. Pat. Nos. 6,322,901 and 6,576,291, andU.S. patent application Ser. No. 60/550,314, each of which isincorporated by reference in its entirety. The method of manufacturing ananocrystal is a colloidal growth process. Colloidal growth occurs byrapidly injecting an M donor and an X donor into a hot coordinatingsolvent. The injection produces a nucleus that can be grown in acontrolled manner to form a nanocrystal. The reaction mixture can begently heated to grow and anneal the nanocrystal. Both the average sizeand the size distribution of the nanocrystals in a sample are dependenton the growth temperature. The growth temperature necessary to maintainsteady growth increases with increasing average crystal size. Thenanocrystal is a member of a population of nanocrystals. As a result ofthe discrete nucleation and controlled growth, the population ofnanocrystals obtained has a narrow, monodisperse distribution ofdiameters. The monodisperse distribution of diameters can also bereferred to as a size. The process of controlled growth and annealing ofthe nanocrystals in the coordinating solvent that follows nucleation canalso result in uniform surface derivatization and regular corestructures. As the size distribution sharpens, the temperature can beraised to maintain steady growth. By adding more M donor or X donor, thegrowth period can be shortened.

The M donor can be an inorganic compound, an organometallic compound, orelemental metal. M is cadmium, zinc, magnesium, mercury, aluminum,gallium, indium or thallium. The X donor is a compound capable ofreacting with the M donor to form a material with the general formulaMX. Typically, the X donor is a chalcogenide donor or a pnictide donor,such as a phosphine chalcogenide, a bis(silyl) chalcogenide, dioxygen,an ammonium salt, or a tris(silyl) pnictide. Suitable X donors includedioxygen, bis(trimethylsilyl) selenide ((TMS)₂Se), trialkyl phosphineselenides such as (tri-n-octylphosphine) selenide (TOPSe) or(tri-n-butylphosphine) selenide (TBPSe), trialkyl phosphine telluridessuch as (tri-n-octylphosphine) telluride (TOPTe) orhexapropylphosphorustriamide telluride (HPPTTe),bis(trimethylsilyl)telluride ((TMS)₂Te), bis(trimethylsilyl)sulfide((TMS)₂S), a trialkyl phosphine sulfide such as (tri-n-octylphosphine)sulfide (TOPS), an ammonium salt such as an ammonium halide (e.g.,NH₄Cl), tris(trimethylsilyl) phosphide ((TMS)₃P), tris(trimethylsilyl)arsenide ((TMS)₃As), or tris(trimethylsilyl) antimonide ((TMS)₃Sb). Incertain embodiments, the M donor and the X donor can be moieties withinthe same molecule.

A coordinating solvent can help control the growth of the nanocrystal.The coordinating solvent is a compound having a donor lone pair that,for example, has a lone electron pair available to coordinate to asurface of the growing nanocrystal. Solvent coordination can stabilizethe growing nanocrystal. Typical coordinating solvents include alkylphosphines, alkyl phosphine oxides, alkyl phosphonic acids, or alkylphosphinic acids, however, other coordinating solvents, such aspyridines, furans, and amines may also be suitable for the nanocrystalproduction. Examples of suitable coordinating solvents include pyridine,tri-n-octyl phosphine (TOP), tri-n-octyl phosphine oxide (TOPO) andtris-hydroxylpropylphosphine (tHPP). Technical grade TOPO can be used.

Size distribution during the growth stage of the reaction can beestimated by monitoring the absorption line widths of the particles.Modification of the reaction temperature in response to changes in theabsorption spectrum of the particles allows the maintenance of a sharpparticle size distribution during growth. Reactants can be added to thenucleation solution during crystal growth to grow larger crystals. Bystopping growth at a particular nanocrystal average diameter andchoosing the proper composition of the semiconducting material, theemission spectra of the nanocrystals can be tuned continuously over thewavelength range of 300 nm to 5 microns, or from 400 nm to 800 nm forCdSe and CdTe. The nanocrystal has a diameter of less than 150 Å. Apopulation of nanocrystals has average diameters in the range of 15 Å to125 Å.

The nanocrystal can be a member of a population of nanocrystals having anarrow size distribution. The nanocrystal can be a sphere, rod, disk, orother shape. The nanocrystal can include a core of a semiconductormaterial. The nanocrystal can include a core having the formula MX,where M is cadmium, zinc, magnesium, mercury, aluminum, gallium, indium,thallium, or mixtures thereof, and X is oxygen, sulfur, selenium,tellurium, nitrogen, phosphorus, arsenic, antimony, or mixtures thereof.

The core can have an overcoating on a surface of the core. Theovercoating can be a semiconductor material having a compositiondifferent from the composition of the core: The overcoat of asemiconductor material on a surface of the nanocrystal can include aGroup II-VI compound, a Group II-V compound, a Group III-VI compound, aGroup III-V compound, a Group IV-VI compound, a Group I-III-VI compound,a Group II-IV-VI compound, and a Group II-IV-V compound, for example,ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO,HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP,InAs, InSb, TlN, TlP, TlAs, TlSb, TlSb, PbS, PbSe, PbTe, or mixturesthereof For example, ZnS, ZnSe or CdS overcoatings can be grown on CdSeor CdTe nanocrystals. An overcoating process is described, for example,in U.S. Pat. No. 6,322,901. By adjusting the temperature of the reactionmixture during overcoating and monitoring the absorption spectrum of thecore, over coated materials having high emission quantum efficienciesand narrow size distributions can be obtained. The overcoating can bebetween 1 and 10 monolayers thick.

The particle size distribution can be further refined by size selectiveprecipitation with a poor solvent for the nanocrystals, such asmethanol/butanol as described in U.S. Pat. No. 6,322,901. For example,nanocrystals can be dispersed in a solution of 10% butanol in hexane.Methanol can be added dropwise to this stirring solution untilopalescence persists. Separation of supernatant and flocculate bycentrifugation produces a precipitate enriched with the largestcrystallites in the sample. This procedure can be repeated until nofurther sharpening of the optical absorption spectrum is noted.Size-selective precipitation can be carried out in a variety ofsolvent/nonsolvent pairs, including pyridine/hexane andchloroform/methanol. The size-selected nanocrystal population can haveno more than a 15% rms deviation from mean diameter, preferably 10% rmsdeviation or less, and more preferably 5% rms deviation or less.

The outer surface of the nanocrystal can include compounds derived fromthe coordinating solvent used during the growth process. The surface canbe modified by repeated exposure to an excess of a competingcoordinating group. For example, a dispersion of the capped nanocrystalcan be treated with a coordinating organic compound, such as pyridine,to produce crystallites which disperse readily in pyridine, methanol,and aromatics but no longer disperse in aliphatic solvents. Such asurface exchange process can be carried out with any compound capable ofcoordinating to or bonding with the outer surface of the nanocrystal,including, for example, phosphines, thiols, amines and phosphates. Thenanocrystal can be exposed to short chain polymers which exhibit anaffinity for the surface and which terminate in a moiety having anaffinity for a suspension or dispersion medium. Such affinity improvesthe stability of the suspension and discourages flocculation of thenanocrystal. Nanocrystal coordinating compounds are described, forexample, in U.S. Pat. No. 6,251,303, which is incorporated by referencein its entirety.

More specifically, the coordinating ligand can have the formula:

wherein k is 2, 3 or 5, and n is 1, 2, 3, 4 or 5 such that k-n is notless than zero; X is O, S, S═O, SO₂, Se, Se═O, N, N═O, P, P═O, As, orAs═O; each of Y and L, independently, is aryl, heteroaryl, or a straightor branched C₂₋₁₂ hydrocarbon chain optionally containing at least onedouble bond, at least one triple bond, or at least one double bond andone triple bond. The hydrocarbon chain can be optionally substitutedwith one or more C₁₋₄ alkyl, C₂₋₄ alkenyl, C₂₋₄ alkynyl, C₁₋₄ alkoxy,hydroxyl, halo, amino, nitro, cyano, C₃₋₅ cycloalkyl, 3-5 memberedheterocycloalkyl, aryl, heteroaryl, C₁₋₄ alkylcarbonyloxy, C₁₋₄alkyloxycarbonyl, C₁₋₄ alkylcarbonyl, or formyl. The hydrocarbon chaincan also be optionally interrupted by —O—, —S—, —N(R^(a))—,—N(R^(a))—C(O)—O—, —O—C(O)—N(R^(a))—, —N(R^(a))—C(O)—N(R^(b))—,—O—C(O)—O—, —P(R^(a))—, or —P(O)(R^(a))—. Each of R^(a) and R^(b),independently, is hydrogen, alkyl, alkenyl, alkynyl, alkoxy,hydroxylalkyl, hydroxyl, or haloalkyl.

An aryl group is a substituted or unsubstituted cyclic aromatic group.Examples include phenyl, benzyl, naphthyl, tolyl, anthracyl,nitrophenyl, or halophenyl. A heteroaryl group is an aryl group with oneor more heteroatoms in the ring, for instance furyl, pyiridyl, pyrrolyl,phenanthryl.

A suitable coordinating ligand can be purchased commercially or preparedby ordinary synthetic organic techniques, for example, as described inJ. March, Advanced Organic Chemistry, which is incorporated by referencein its entirety.

Transmission electron microscopy (TEM) can provide information about thesize, shape, and distribution of the nanocrystal population. PowderX-ray diffraction (XRD) patterns can provide the most completeinformation regarding the type and quality of the crystal structure ofthe nanocrystals. Estimates of size are also possible since particlediameter is inversely related, via the X-ray coherence length, to thepeak width. For example, the diameter of the nanocrystal can be measureddirectly by transmission electron microscopy or estimated from X-raydiffraction data using, for example, the Scherrer equation. It also canbe estimated from the UV/V is absorption spectrum.

Example

One example of an optical structure including nanocrystals is describedbelow.

A conventional 0.5 mm plastic fiber optic element was stripped of itssheath and cladding. The cladding was removed by soaking the fiber inacetone and wiping the fiber to remove the dissolved cladding material.Red luminescent semiconductor nanocrystals (quantum dots) in an ethanolsolution were then applied to the exterior of the stripped fiber. Thenanocrystal layer was allowed to dry. A conventional fiber opticend-coupled 475 nm light emitting diode was then attached to the fiberand turned on. FIG. 3 shows the spectrum of light emitted from thefiber. It is clear from the spectrum that the evanescent wave couples tothe nanocrystals, which then emitted red light. Some of the excitationlight was also emitted from the fiber, likely because of the surfaceroughness of the fiber that scatters the fiber-guided blue light. FIG. 4shows a photograph of the light emitting structure. Red light from theevanescent wave coupled nanocrystals is easily visible.

Other embodiments are within the scope of the following claims.

The invention claimed is:
 1. An optical structure comprising ananocrystal on a surface of an optical waveguide, the nanocrystal beingpositioned to be optically coupled to an optical field propagatingthrough the optical waveguide and emit light from the surface of thewaveguide, wherein the optical waveguide is tuned to select the amountof excitation wavelength the nanocrystal encounters, wherein theintensity of the excitation wavelength propagating through the opticalwaveguide that each nanocrystal in the plurality of nanocrystalsencounters is determined by the position of each nanocrystal on thesurface of the optical waveguide.
 2. The optical structure of claim 1,wherein the waveguide is an optical fiber.
 3. The optical structure ofclaim 1, wherein the waveguide is a planar waveguide.
 4. The opticalstructure of claim 1, wherein the nanocrystal is a semiconductornanocrystal.
 5. The optical structure of claim 2, wherein the opticalfiber has a cladding layer that allows light to escape at a selectedamount along the length of the fiber.
 6. The optical structure of claim4, wherein the semiconductor nanocrystal includes a core including afirst semiconductor material.
 7. The optical structure of claim 6,wherein the semiconductor nanocrystal includes an overcoating on asurface of the core including a second semiconductor material.
 8. Theoptical structure of claim 1, further comprising a plurality ofnanocrystals distributed at a first portion of the surface.
 9. Theoptical structure of claim 8, further comprising a plurality ofnanocrystals distributed at a second portion of the surface.
 10. Theoptical structure of claim 9, wherein the plurality of nanocrystalsdistributed at the first portion of the surface has a compositiondifferent from the plurality of nanocrystals distributed at the secondportion of the surface.
 11. A light emitting structure comprising: alight source arranged to introduce light including an excitationwavelength into an optical waveguide; and a nanocrystal on a surface ofthe optical waveguide, the nanocrystal being positioned to be opticallycoupled to an optical field propagating through the optical waveguideand capable of absorbing the excitation wavelength of light and emittingan emission wavelength of light and emit light from the surface of thewaveguide, wherein the optical waveguide is tuned to select the amountof excitation wavelength the nanocrystal encounters, wherein theintensity of the excitation wavelength propagating through the opticalwaveguide that each nanocrystal in the plurality of nanocrystalsencounters is determined by the position of each nanocrystal on thesurface of the optical waveguide.
 12. The light emitting structure ofclaim 11, wherein the waveguide is an optical fiber.
 13. The lightemitting structure of claim 11, wherein the waveguide is a planarwaveguide.
 14. The light emitting structure of claim 11, wherein thenanocrystal is a semiconductor nanocrystal.
 15. The light emittingstructure of claim 12, wherein the optical fiber has a cladding layerthat allows light to escape at a selected amount along the length of thefiber.
 16. The light emitting structure of claim 14, wherein thesemiconductor nanocrystal includes a core including a firstsemiconductor material.
 17. The light emitting structure of claim 16,wherein the semiconductor nanocrystal includes an overcoating on asurface of the core including a second semiconductor material.
 18. Thelight emitting structure of claim 11, further comprising a plurality ofnanocrystals distributed at a first portion of the surface.
 19. Thelight emitting structure of claim 18, further comprising a plurality ofnanocrystals distributed at a second portion of the surface.
 20. Thelight emitting structure of claim 19, wherein the plurality ofnanocrystals distributed at the first portion of the surface has acomposition different from the plurality of nanocrystals distributed atthe second portion of the surface.
 21. A method of producing lightcomprising: introducing light from a light source including anexcitation wavelength into an optical waveguide, the excitationwavelength propagating through the optical waveguide and opticallycoupling to a nanocrystal on a surface of the optical waveguide and emitlight from the surface of the waveguide, the nanocrystal absorbing theexcitation wavelength and emitting an emission wavelength from thesurface, and tuning the optical waveguide to select the amount ofexcitation wavelength the nanocrystal encounters, wherein the intensityof the excitation wavelength propagating through the optical waveguidethat each nanocrystal in the plurality of nanocrystals encounters isdetermined by the position of each nanocrystal on the surface of theoptical waveguide.
 22. The method of claim 21, wherein the waveguide isan optical fiber.
 23. The method of claim 21, wherein the nanocrystal isa semiconductor nanocrystal.
 24. The method of claim 21, furthercomprising modifying the surface of the optical waveguide to increasecoupling between the optical field and the nanocrystal to allow light toescape at a selected amount at selected locations.
 25. The method ofclaim 21, wherein the semiconductor nanocrystal includes a coreincluding a first semiconductor material.
 26. The method of claim 21,wherein the excitation wavelength propagates through the opticalwaveguide and optically couples to a plurality of nanocrystals on afirst portion of a surface of the optical waveguide.
 27. The method ofclaim 26, wherein the excitation wavelength propagates through theoptical waveguide and optically couples to a plurality of nanocrystalson a second portion of the surface.
 28. The light emitting structure ofclaim 27, wherein the plurality of nanocrystals distributed at the firstportion of the surface has a composition different from the plurality ofnanocrystals distributed at the second portion of the surface.
 29. Thelight emitting structure of claim 27, wherein the plurality ofnanocrystals distributed at the first portion of the surface has anemission wavelength different from the plurality of nanocrystalsdistributed at the second portion of the surface.
 30. A method of makingan optical structure comprising: placing a nanocrystal on a surface ofan optical waveguide in a position to optically couple the nanocrystalto an optical field propagating through the optical waveguide and emitlight from the surface of the waveguide, and tuning the opticalwaveguide to select the amount of excitation wavelength the nanocrystalencounters, wherein the intensity of the excitation wavelengthpropagating through the optical waveguide that each nanocrystal in theplurality of nanocrystals encounters is determined by the position ofeach nanocrystal on the surface of the optical waveguide.
 31. The methodof claim 30, wherein placing includes dip coating, drop coating, spincoating, painting or printing the nanocrystal on the surface.
 32. Themethod of claim 30, further comprising processing the surface of theoptical waveguide prior to placing the nanocrystal.
 33. An opticalstructure comprising a nanocrystal on a surface of an optical waveguide,the nanocrystal being positioned to be optically coupled to an opticalfield propagating through the optical waveguide and emit light from thesurface of the waveguide, a plurality of nanocrystals distributed at afirst portion of the surface, and a plurality of nanocrystalsdistributed at a second portion of the surface, wherein the opticalwaveguide is tuned to select the amount of excitation wavelength thenanocrystal encounters, wherein the intensity of the excitationwavelength propagating through the optical waveguide that eachnanocrystal in the plurality of nanocrystals encounters is determined bythe position of each nanocrystal on the surface of the opticalwaveguide.
 34. The optical structure of claim 33, wherein the pluralityof nanocrystals distributed at the first portion of the surface has acomposition different from the plurality of nanocrystals distributed atthe second portion of the surface.
 35. A light emitting structurecomprising: a light source arranged to introduce light including anexcitation wavelength into an optical waveguide; and a nanocrystal on asurface of the optical waveguide, the nanocrystal being positioned to beoptically coupled to an optical field propagating through the opticalwaveguide and capable of absorbing the excitation wavelength of lightand emitting an emission wavelength of light and emit light from thesurface of the waveguide, a plurality of nanocrystals distributed at afirst portion of the surface, and a plurality of nanocrystalsdistributed at a second portion of the surface, wherein the opticalwaveguide is tuned to select the amount of excitation wavelength thenanocrystal encounters, wherein the intensity of the excitationwavelength propagating through the optical waveguide that eachnanocrystal in the plurality of nanocrystals encounters is determined bythe position of each nanocrystal on the surface of the opticalwaveguide.
 36. The optical structure of claim 34, wherein the pluralityof nanocrystals distributed at the first portion of the surface has acomposition different from the plurality of nanocrystals distributed atthe second portion of the surface.
 37. The optical structure of claim 1,wherein light not absorbed by the nanocrystal continues circling thewaveguide.
 38. The light emitting structure of claim 11, wherein lightnot absorbed by the nanocrystal continues circling the waveguide. 39.The method of producing light in claim 21, wherein light not absorbed bythe nanocrystal continues circling the waveguide.
 40. The method ofmaking an optical structure of claim 30, wherein light not absorbed bythe nanocrystal continues circling the waveguide.
 41. The opticalstructure of claim 33, wherein light not absorbed by the nanocrystalcontinues circling the waveguide.
 42. The light emitting structure ofclaim 35, wherein light not absorbed by the nanocrystal continuescircling the waveguide.